The present invention relates to an information recording medium, and more particularly to an optical information recording medium.
A super-resolution optical system is used to increase the recording density of an optical information recording medium. The general structure of the super-resolution optical system is disclosed in the following references:
Yamanaka et al., "High Density Optical Recording by Super-resolution system", International Symposium on Optical Memory 1989, 27D--17, pp. 99-100, Sept. 1989, Technical Digest;
Fukuhisa et al., "Reproducing C/N Characteristic of High Density Magneto-Optic Disk Using Super Resolution Head", Extended Abstracts of the 39th Meeting of the Japanese Society of Applied Physics and Related Societies, 31p-L-6, p. 1002, March, 1992; and
Ichiura et al., "High Density Video Disc Using Super-resolution and Green Laser", International Symposium on Optical Memory and Optical Data Storage '93, Tul. 4, pp. 10-11, July 1993.
In a super-resolution optical system, diffraction means (e.g., a shield plate or a double rhomb prism) is placed between a light source and a medium. The diffraction means forms a diffraction pattern of the light on the medium. The diameter of the center spot (main lobe) of the diffraction pattern is approximately 20% smaller than that of the smallest spot formed without the diffraction means. The reduction of the spot diameter increases the recording density of the medium by about 1.5 times.
The super-resolution optical system is used with a conventional optical information recording medium based on the phase-contrast technique. Examples of the conventional optical recording medium are disclosed in the following references.
A conventional medium including a Pb-Te-Se film is disclosed in M. Terao, et al. "Oxidation Resistance of Pb-Te-Se Optical Recording Film", Journal of Applied Physics 62 (3), p.1029, 1987.
A conventional medium including a Te-C film is disclosed in Katsutaro Ichihara, Hideki Okawa, "Optical Disk Medium", Electronic Ceramics Vol. 18 (90), p.5, November 1987, and M. Mashira, N. Yasuda, "Amorphous Te-C Films for an Optical Disk", Proceedings SPIE Optical Disk Technology Vol. 329, pp.190-194, 1982.
A conventional medium including CS.sub.2 -Te film is disclosed in H. Yamazaki, et al., "Plasma Polymerized CS.sub.2 -Te Film for Laser Beam Memory", Review of Electrical Communication Laboratories Vol. 32 No. 2, pp.260-266, 1984.
A conventional medium including an organic recording film is disclosed in a Japanese Unexamined Patent Publication sho-62-119755.
The conventional phase-contrast medium has a mirror surface and a bump protruding from the mirror surface (hereinafter referred to as a mirror region and a pit region). Typically, the height of the bump is one-quarter the wavelength of the light.
In the conventional phase-contrast medium, contrast, which represents "bits" of information, is produced by the destructive interference of light waves. Therefore, the mirror region and the pit region have nearly the same reflectivity. When the light spot is on a planar part of the mirror region, the light is simply reflected from the mirror region. When the light spot is on the pit region, the light is reflected from the pit region as well as from the mirror region. The light wave reflected from the pit region and the light wave reflected from the mirror region are in phase opposition. Thus, the destructive interference of the two light waves occurs to suppress light reflection in the pit region.
However, the combination of the super-resolution optical system and the conventional media causes the following problem.
In the super-resolution optical system, the diffraction pattern formed on the medium includes a main lobe (the center spot) and side lobes on the both sides of the main lobe. The medium reflects the side lobes as well as the main lobe. The reflected beams of the main lobe and the side lobes interfere with each other to cause an edge shift in the reproduced signal. The edge shift distorts the reproduced signal to increase the jitter in the reproduced signal.
A filter (e.g., a slit or a pin-hole) can be incorporated in the super-resolution optical system to remove the reflected beam of the side lobes. However, the filter cannot remove the reflected beam of the side lobes sufficiently to prevent distortion and jitter in the reproduced signal.